A polyethylene-based resin foam is generally used as a packing material owing to its high resilience and excellent oil and impact resistance. The polyethylene-based resin foam, however, has drawbacks that its stiffness and compressive strength are low. On the other hand, a polystyrene-based resin foam is excellent in stiffness, but has a drawback that it is brittle.
To overcome such drawbacks, Japanese Examined Patent Publication No. SHO 51 (1976)-46138 and Japanese Unexamined Patent Publication No. SHO 62 (1987)-59642 disclose a method for obtaining expandable particles of a styrene-modified polyethylene-based resin by impregnating a styrene monomer into a polyethylene-based resin for polymerization.
Examples of the polyethylene-based resin used in the method substantially include low-density polyethylene, high-density polyethylene, and an ethylene-vinyl acetate copolymer. In most cases, polyethylene is cross-linked to improve its moldability and physical properties of a molded product. The cross-linking of polyethylene increases the strength of foam membrane and also increases the tension of the membrane at expansion molding, so that the membrane is prevented from breaking and an expansion ratio can be increased.
Consequently, an expanded molded article with a fine appearance is provided and the impact strength of the expanded molded article can be increased. This method, however, requires that pre-crosslinked polyethylene be used or a cross-linking step be provided for cross-linking polyethylene after a temperature is furthermore raised at the end of the polymerization of styrene monomer.
To solve the above-mentioned problems, Japanese Patent No. 2668384 also discloses a method for obtaining an expanded molded article of a modified polyethylene-based resin excellent in stiffness and impact resistance. In this method, 100 parts by weight of non-crosslinked linear low-density polyethylene-based resin particles, 5 to 300 parts by weight of a vinyl aromatic monomer, and 1 to 3 parts by weight of a polymerization initiator relative to 100 parts by weight of the vinyl aromatic monomer are dispersed in an aqueous medium. Then, a suspension thus obtained is heated at such a temperature that polymerization of the monomer does not substantially take place for impregnation of the monomer into an inside and a surface of the polyethylene-based resin particles. Subsequently, the temperature of the suspension is raised to polymerize the monomer, as a result, the expanded molded article of the modified polyethylene-based resin is obtained by micro-dispersion of a vinyl aromatic polymer in polyethylene.
In Examples of the above-mentioned patent, a styrene monomer is added to linear low-density polyethylene-based resin particles having a melting point of 122° C. for polymerization at 115° C. (the melting point of the resin particles is not specified in Examples, but the present inventors confirmed from the product name of the resin particles described in Examples that the particles have the above-mentioned melting point). The polymerization at this temperature often results in graft polymerization of styrene monomer on a polyethylene chain. Consequently, though a resultant resin is not cross-linked, the graft polymerization of polystyrene on the polyethylene chain occurs, and thereby a fine expanded molded article can be provided. Hereinafter, the term “graft polymer” means a gel component containing polystyrene, and the term “crosslinked polymer” means a gel component substantially not containing polystyrene.
In the above-mentioned patent, an amount of the styrene monomer is 5 to 300 parts by weight relative to 100 parts by weight of polyethylene, and when more than 300 parts by weight of the styrene monomer is impregnated into polyethylene, there is a problem that a large amount of polymer powder of polystyrene is generated.
Where a styrene monomer is impregnated into polyethylene-based resin particles for polymerization to obtain a resin which is subsequently impregnated with a volatile blowing agent and molded by heating to obtain an expanded molded article, polyethylene needs to be cross-linked for enhancing impact resistance of the expanded molded article and for reducing a size variation of the expanded molded article after heating. In other words, the cross-linking of polyethylene is required for the expanded molded article to have heat resistance and higher stiffness. However, there has been a problem that the cross-linking involves an increase in cost due to the use of a crosslinker and an increase in a production step.
Accordingly, there has been desired development of expandable particles of a styrene-modified linear low-density polyethylene-based resin capable of providing an expanded molded article having sufficient strength, in which a ratio of styrene monomer to polyethylene can be widely changed.